3 files changed, 58 insertions, 60 deletions

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 931a619407bf..b8ba85544721 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -30,15 +30,6 @@ static inline s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder)
 }
 
 /**
- * div64_u64_rem - unsigned 64bit divide with 64bit divisor
- */
-static inline u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
-{
-        *remainder = dividend % divisor;
-        return dividend / divisor;
-}
-
-/**
  * div64_u64 - unsigned 64bit divide with 64bit divisor
  */
 static inline u64 div64_u64(u64 dividend, u64 divisor)
@@ -70,16 +61,8 @@ static inline u64 div_u64_rem(u64 dividend, u32 divisor, u32 *remainder)
 extern s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder);
 #endif
 
-#ifndef div64_u64_rem
-extern u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder);
-#endif
-
 #ifndef div64_u64
-static inline u64 div64_u64(u64 dividend, u64 divisor)
-{
-        u64 remainder;
-        return div64_u64_rem(dividend, divisor, &remainder);
-}
+extern u64 div64_u64(u64 dividend, u64 divisor);
 #endif
 
 #ifndef div64_s64
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index ea32f02bf2c3..cc2dc3eea8a3 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -506,34 +506,47 @@ void account_idle_ticks(unsigned long ticks)
 }
 
 /*
- * Perform (stime * rtime) / total with reduced chances
- * of multiplication overflows by using smaller factors
- * like quotient and remainders of divisions between
- * rtime and total.
+ * Perform (stime * rtime) / total, but avoid multiplication overflow by
+ * loosing precision when the numbers are big.
  */
 static cputime_t scale_stime(u64 stime, u64 rtime, u64 total)
 {
-        u64 rem, res, scaled;
+        u64 scaled;
 
-        if (rtime >= total) {
-                /*
-                 * Scale up to rtime / total then add
-                 * the remainder scaled to stime / total.
-                 */
-                res = div64_u64_rem(rtime, total, &rem);
-                scaled = stime * res;
-                scaled += div64_u64(stime * rem, total);
-        } else {
-                /*
-                 * Same in reverse: scale down to total / rtime
-                 * then substract that result scaled to
-                 * to the remaining part.
-                 */
-                res = div64_u64_rem(total, rtime, &rem);
-                scaled = div64_u64(stime, res);
-                scaled -= div64_u64(scaled * rem, total);
+        for (;;) {
+                /* Make sure "rtime" is the bigger of stime/rtime */
+                if (stime > rtime) {
+                        u64 tmp = rtime; rtime = stime; stime = tmp;
+                }
+
+                /* Make sure 'total' fits in 32 bits */
+                if (total >> 32)
+                        goto drop_precision;
+
+                /* Does rtime (and thus stime) fit in 32 bits? */
+                if (!(rtime >> 32))
+                        break;
+
+                /* Can we just balance rtime/stime rather than dropping bits? */
+                if (stime >> 31)
+                        goto drop_precision;
+
+                /* We can grow stime and shrink rtime and try to make them both fit */
+                stime <<= 1;
+                rtime >>= 1;
+                continue;
+
+drop_precision:
+                /* We drop from rtime, it has more bits than stime */
+                rtime >>= 1;
+                total >>= 1;
         }
 
+        /*
+         * Make sure gcc understands that this is a 32x32->64 multiply,
+         * followed by a 64/32->64 divide.
+         */
+        scaled = div_u64((u64) (u32) stime * (u64) (u32) rtime, (u32)total);
         return (__force cputime_t) scaled;
 }
 
@@ -545,7 +558,7 @@ static void cputime_adjust(struct task_cputime *curr,
                            struct cputime *prev,
                            cputime_t *ut, cputime_t *st)
 {
-        cputime_t rtime, stime, total;
+        cputime_t rtime, stime, utime, total;
 
         if (vtime_accounting_enabled()) {
                 *ut = curr->utime;
@@ -568,13 +581,21 @@ static void cputime_adjust(struct task_cputime *curr,
          */
         rtime = nsecs_to_cputime(curr->sum_exec_runtime);
 
-        if (!rtime) {
-                stime = 0;
-        } else if (!total) {
-                stime = rtime;
-        } else {
+        /*
+         * Update userspace visible utime/stime values only if actual execution
+         * time is bigger than already exported. Note that can happen, that we
+         * provided bigger values due to scaling inaccuracy on big numbers.
+         */
+        if (prev->stime + prev->utime >= rtime)
+                goto out;
+
+        if (total) {
                 stime = scale_stime((__force u64)stime,
                                     (__force u64)rtime, (__force u64)total);
+                utime = rtime - stime;
+        } else {
+                stime = rtime;
+                utime = 0;
         }
 
         /*
@@ -583,8 +604,9 @@ static void cputime_adjust(struct task_cputime *curr,
          * Let's enforce monotonicity.
          */
         prev->stime = max(prev->stime, stime);
-        prev->utime = max(prev->utime, rtime - prev->stime);
+        prev->utime = max(prev->utime, utime);
 
+out:
         *ut = prev->utime;
         *st = prev->stime;
 }
diff --git a/lib/div64.c b/lib/div64.c
index 3af5728d95fd..a163b6caef73 100644
--- a/lib/div64.c
+++ b/lib/div64.c
@@ -79,10 +79,9 @@ EXPORT_SYMBOL(div_s64_rem);
 #endif
 
 /**
- * div64_u64_rem - unsigned 64bit divide with 64bit divisor and 64bit remainder
+ * div64_u64 - unsigned 64bit divide with 64bit divisor
  * @dividend:   64bit dividend
  * @divisor:    64bit divisor
- * @remainder:  64bit remainder
  *
  * This implementation is a modified version of the algorithm proposed
  * by the book 'Hacker's Delight'.  The original source and full proof
@@ -90,33 +89,27 @@ EXPORT_SYMBOL(div_s64_rem);
  *
  * 'http://www.hackersdelight.org/HDcode/newCode/divDouble.c.txt'
  */
-#ifndef div64_u64_rem
-u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
+#ifndef div64_u64
+u64 div64_u64(u64 dividend, u64 divisor)
 {
         u32 high = divisor >> 32;
         u64 quot;
 
         if (high == 0) {
-                u32 rem32;
-                quot = div_u64_rem(dividend, divisor, &rem32);
-                *remainder = rem32;
+                quot = div_u64(dividend, divisor);
         } else {
                 int n = 1 + fls(high);
                 quot = div_u64(dividend >> n, divisor >> n);
 
                 if (quot != 0)
                         quot--;
-
-                *remainder = dividend - quot * divisor;
-                if (*remainder >= divisor) {
+                if ((dividend - quot * divisor) >= divisor)
                         quot++;
-                        *remainder -= divisor;
-                }
         }
 
         return quot;
 }
-EXPORT_SYMBOL(div64_u64_rem);
+EXPORT_SYMBOL(div64_u64);
 #endif
 
 /**